Semiconductor laser

Abstract: A semiconductor laser according to the present invention comprises a first cladding layer (2) of a first conductivity type on a substrate, an active layer (3) laminated on the first cladding layer, a second cladding layer (4) of a second conductivity type laminated on the active layer, and a current confinement layer (7) on the second cladding layer, in which the first cladding layer, the active layer and the second cladding layer are made of the II-VI group compound semiconductor and the current confinement layer (7) is made of a metal oxide whose thermal conductivity is equal to or larger than 0.01 cal/cm·s·°C. According to the present invention having the above-mentioned construction, a silicon layer (12) is provided between the current confinement layer and an electrode. Further, according to the present invention having the above-mentioned construction, the current confinement layer is provided on an upper portion of a p type semiconductor layer. According to the present invention, particularly in a semiconductor laser made of the II-VI group compound semiconductor of ZnSe system of the like, heat-radiation characteristic thereof can be improved and long-time operation thereof is made possible.

Description: BACKGROUND OF THE INVENTION The present invention relates to a semiconductor laser and particularly to a semiconductor laser whose active layer is made of a II-VI group compound semiconductor material. It is difficult to obtain a satisfactory ohmic contact at a p side in a semiconductor laser in which a II-VI group compound semiconductor material of ZnSe system or the like is used. Therefore, the prior art is encountered by such disadvantage that generation of heat at a contact portion causes deterioration of an electrode, rise in temperature of an element makes long-time operation difficult, or the like. Construction of the ZnSe system semiconductor laser as an example will be explained with reference to FIGS. 1A to 1C showing manufacturing processes thereof. In this case, first, as shown in FIG. 1A, a first cladding layer 2 made of an n type ZnMgSSe or the like, an active layer 3 made of an undoped ZnCdSe or the like, a second cladding layer 4 made of a p type ZnMgSSe or the like, a buffer layer 5 made of a p type ZnSe or the like and further a contact layer 6 having a p type ZnSe/ZnTe superlattice structure are successively epitaxially grown on a substrate 1 made of an n type GaAs or the like. A photoresist 10 is coated on the contact layer 6 and then subjected to patterning by pattern exposure and development in a striped fashion so as to cover a region where a striped current path is to be formed. With using the photoresist 10 as a resist mask, the contact layer 6 is subjected to the patterning by chemical etching or the like so as to have a striped shape. At this time, until a part of each of the buffer layers 5 at both sides of the contact layer 6 is removed, the etching is performed. Then, after the photoresist 10 is removed and then a polyimide 11 is coated on the whole surface, as shown in FIG. 1B, only the contact layer 6 is exposed by a proper photolithography technique. Next, as shown in FIG. 1C, an electrode 8 formed of a multilayer film of Pd/Pt/Au or the like is formed by vacuum evaporation or the like so as to completely cover the polyimide 11 and the contact layer 6. Reference numeral 9 represents an electrode which is made of In or the like and previously formed by the coating on a rear surface of the substrate 1, for example, or the like. As described above, the polyimide 11 blocks the current and the region sandwiched by the polyimide 11 is set as the striped current path, thereby constructing a semiconductor laser of a gain waveguide type. The polyimide 11 has such advantages that it has excellent property in adhesion to ZnSe and a p side electrode, can be formed as a film by a process at a comparatively low temperature, and is prevented from damaging crystallinity of another portions. However, the above-mentioned semiconductor laser of gain waveguide type has such disadvantage that since most injected current is concentrated on a portion just beneath an electrode having large contact resistance, it cannot be avoided that Joule heat generated around the electrode deteriorates the electrode portion and raise temperature of the whole of a laser element. An other II-VI compound semiconductor laser diode is known from WO-A-92/21170. There can be considered a countermeasure thereagainst such that the p side electrode is located at a lower portion of the element in order to suppress rise of temperature of the element and is mounted on a heat sink having very high thermal conductivity, while there is then the problem that since the polyimide 11 is provided on a portion occupying a comparatively large area between the p side electrode and the cladding layer, the heat cannot be effectively exhausted. From Research Disclosure (1993)February, no. 346, p. 81, Emsworth, GB it is known to use Al 2 O 3 instead of polyimide as an insulating layer of a II-VI semiconductor laser diode. SUMMARY OF THE INVENTION According to the present invention, particularly in the II-VI group compound semiconductor laser of ZnSe system or the like, heat-radiation characteristic can be further improved, and long-time operation thereof is made possible. A semiconductor laser according to the present invention comprises the features of appended claim 1. Further according to the present invention having the above-mentioned construction, the current confinement layer is provided on an upper portion of a p type semiconductor layer. BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1A, 1B and 1C are manufacturing process diagrams of one example of a semiconductor laser; FIG. 2 is a schematically enlarged and cross-sectional view of an embodiment according to the present invention; FIGS. 3A, 3B and 3C are manufacturing process diagrams of the embodiment according to the present invention; and FIG. 4 is a schematically enlarged and cross-sectional view of another embodiment according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Through study and research of the inventor of the present invention, it is found that as shown in FIG. 2 which is a schematically enlarged and cross-sectional view of one embodiment of the present invention, when in constructing a semiconductor laser whose active layer is made of II-VI group compound semiconductor, a metal oxide having satisfactory thermal conductivity in which its thermal conductivity at 100 °C is equal to or larger than 0.01 cal/cm·s·°C is used particularly as a material of a current confinement layer 7 thereof to thereby obtain sufficient heat-radiation characteristic and enable an element to be operated for a long period of time. A silicon layer is provided between the current confinement layer 7 made of such metal oxide and an electrode 8 thereon, whereby adhesion between the current confinement layer 7 made of the metal oxide and the electrode 8 can be improved and the peeling off or the like can be prevented from being caused to thereby provide a highly reliable semiconductor laser. Such current confinement layer 7, as shown in FIGS. 3A to 3C which show a manufacturing process of one example thereof, is formed in a self-alignment fashion with using a resist 10 on a striped current path as a mask, whereby positional displacement thereof relative to a contact layer 6 can be suppressed and reduction of the number of manufacturing processes and improvement of productivity can be achieved. An embodiment according to the present invention will hereinafter be explained with reference to the accompanying drawings. This example is the case where a ZnSe system compound semiconductor laser is constructed on a GaAs substrate, and one example of a manufacturing method thereof will first be explained in detail with reference to FIGS. 3A to 3C. As shown in FIG. 3A, reference numeral 1 represents a substrate made of an n type GaAs, for example, in which an electrode 9 made of, for example, In or the like is coated as an n side electrode on a rear surface thereof and formed by the heating and the melting in the succeeding epitaxial growth process. On the substrate 1, a first cladding layer 2 made of an n type ZnMgSSe or the like, an active layer 3 made of undoped ZnCdSe or the like, a second cladding layer 4 made of a p type ZnMgSSe or the like, a buffer layer 5 made of a p type ZnSe or the like and further a contact layer 6 having a p type ZnSe/ZnTe superlattice structure, for example, are successively epitaxially grown by an MOCVD (metal organic chemical vapor deposition), an MBE (molecular beam epitaxy) or the like. Next, the photoresist 10 is coated on the contact layer 6 and then subjected to patterning by pattern exposure and development in a striped fashion so as to cover a region where a striped current path is to be formed. With using the photoresist 10 as a resist mask, the contact layer 6 is subjected to the patterning by chemical etching or the like so as to have a striped shape. At this time, until a part of each of the buffer layers 5 at both sides of the contact layer 6 is removed, the etching is performed. Next, as shown in FIG. 3B, with the photoresist 10 being kept left, the current confinement layer 7 made of a metal oxide whose thermal conductivity at 100°C is equal to or larger than 0.01 cal/cm·s·°C, for example, Al 2 O 3 which does not form part of the invention and whose thermal conductivity is 0.07 cal/cm·s·°C (a value at 100 °C, a value at 100 °C of the thermal conductivity will hereinafter be shown as a value of the thermal conductivity) is deposited by the vacuum evaporation or the like on the whole surface. As shown in FIG. 3C, after the photoresist and the Al 2 O 3 thereon are removed to expose the contact layer 6, as shown in FIG. 2, the electrode 8 formed of a multilayer film of Pd/Pt/Au or the like is formed on the whole surface by vacuum evaporation or the like. Thus, there is constructed a semiconductor laser of gain waveguide type in which a region surrounded by the current confinement layer 7 made of the above-mentioned metal oxide is formed as a striped current path. In this case, a photolithography process is required once, and the current confinement layer 7 can be formed in self-alignment fashion simultaneously with the patterning of the contact layer 6, so that positional displacement between the contact layer 6 and the current confinement layer 7 can be suppressed. That is, when a polyimide is used as the current confinement layer on the p type semiconductor layer in the prior art, the photolithography is required twice as explained in the above FIGS. 1A to 1C which show a manufacturing process thereof, while as described above, according to the present invention, the number of photolithography processes can be reduced to one, hence the number of manufacturing processes can be reduced, and further productivity can be improved. A silicon layer 12 is provided between the current confinement 7 and the electrode 8 as shown in FIG. 4, whereby adhesion at this portion can be made sufficiently large. A forming method of the silicon layer 12 proceeds in such a manner that in the above-mentioned process shown in FIG. 3B, for example, after the current confinement layer 7 made of the metal oxide is deposited by the vacuum evaporation, the silicon layer 12 is deposited by the vacuum evaporation on the whole surface and then can be formed in self-alignment fashion by similarly removing the photoresist. The silicon is high in the thermal conductivity as compared with the metal oxide such as Al 2 O 3 or the like, so that drastic deterioration of the heat-radiation characteristic can be avoided as compared with the case where only the metal oxide is used. Besides the above-mentioned Al 2 O 3 example which is useful for understanding the invention, it is sufficient for a material of the current confinement layer 7 that the thermal conductivity is equal to or larger than 0.01 cal/cm·s·°C. As a material which can have property of self-alignment, that is, can be deposited by the vacuum evaporation, there can be used a material such as MgO whose thermal conductivity is 0.09 cal/cm·s·°C, BeO whose thermal conductivity is 0.53 cal/cm·s·°C, or the like. The thermal conductivity of the polyimide is within the range from 0.0003 to 0.0005 cal/cm·s·°C. An insulating material whose thermal conductivity is higher than that of the polyimide is SiO 2 , for example, whose thermal conductivity is 0.005 cal/cm·s·°C. When SiO 2 is employed as the current confinement layer of the semiconductor laser having the active layer formed of the above-mentioned II-VI group compound semiconductor, sufficient heat-radiation characteristic could not be obtained. Accordingly, in the present invention, the metal oxide whose thermal conductivity is equal to or larger than 0.01 cal/cm·s·°C is used. As described above, according to the present invention, in the semiconductor laser made of II-VI group compound semiconductor of ZnSe system or the like, the heat-radiation characteristic thereof can be improved and the longtime operation thereof is made possible. The number of the lithographic processes in the manufacture thereof can be reduced to one, whereby the number of the manufacturing processes can be reduced and the productivity thereof can be improved. Further, the silicon layer is provided between the current confinement layer and the electrode, whereby the adhesion property thereof can be improved and hence more reliable semiconductor laser can be provided. Having described preferred embodiments of the invention with reference to the drawings, it is to be understood that the invention is not limited to that precise embodiments and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the novel concepts of the invention as defined in the appended claims.

Claims: A semiconductor laser comprising: a semiconductor substrate (1); a first cladding layer (2) of a first conductive type disposed on said semiconductor substrate; an active layer (3) disposed on said first cladding layer, a second cladding layer (4) of a second conductivity type disposed on said active layer; and a current confinement layer (7) on said second cladding layer; an electrode (8) disposed on the confinement layer; wherein said first cladding layer, said second cladding layer and said active layer are made of II-VI group semiconductor compounds; characterised in that said current confinement layer (7) is made of BeO or MgO and in that a silicon layer is provided between said current confinement layer and said electrode. The semiconductor laser according to Claim 1, wherein said second conductivity is p-type.

Inventors: NAKAYAMA NORIKAZU, NAKAYAMA, NORIKAZU, NAKAYAMA, NORIKAZU, C/O SONY CORPORATION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1994
Main classification: H01S3
IPC codes: H01S5